Organisms and Population Test 11

In nature, a given habitat has resources to support a certain number of individuals of a population, beyond which no further growth is possible. This limit is called as nature's carrying capacity (K) for that species in that habitat.

S-shaped growth curve is also called Verhulst-Pearl logistic curve and is represented by the following equation :

where dN/dt= rate of change in population size, r= intrinsic rate of natural increase, N= population density, K= carrying capacity and 

Since resources of growth for most animal populations are finite and become limiting sooner or later, the logistic growth model is considered as more realistic.

A population with large number of older individuals than younger ones is likely to decline since older individuals do not take part in reproduction.

Predation is an interaction between members of two species in which members of one species capture, kill and eat up members of other species. The former are called predators and the latter are called preys.

Parasitism is a relationship between two living organisms of different species in which one organism called parasite obtains its food directly from another living organism called host. The parasite is usually smaller as compared to its host. It spends a part or whole of its life on or in the body of the host. Obligate parasites obtain their nutrition from living organisms only.

The evolution of complementary adaptations in two species caused by the selection pressures that each exerts on the other is called co-evolution. It is common in symbiotic associations e.g., many insect-pollinated plants have evolved flowers whose shapes, colours, etc., make them attractive to particular insects, at the same time the pollinating insects have evolved sense organs and mouth parts specialised for quickly locating and extracting nectar from particular species of plants.

According to the mechanism of resource partitioning (supported by Mac Arthur), if two species compete for the same resource, they could avoid competition by choosing, for instance, different times for feeding or different foraging patterns. Two species evolve to become different too reduce competition, so that species can co-exist. 

In mutualism, both species are benefitted. Gut bacteria are dependent on us for food. They release certain enzymes which help us in digestion.

If mitochondria are still considered to be separate organisms, they would form mutualistic relationship with us. This is because they would get the resources from us and in turn produce energy for various activities of the body.

In competition, both species are harmed. In predation and parasitism, one species derives benefit and the other one is harmed. Commensalism is an interaction in which one species is benefitted and other one remains unaffected.

Brood parasitism in birds is an example of parasitism in which the parasitics bird lays its eggs in the nest of its host and the host incubates them. This way, the bird can save the energy that it would have spent in incubation and can use that energy if foraging and other activities. During the course of evolution, the eggs of the parasite bird have evolved to resemble the host's eggd in size and colour to reduce the chances of the host bird detecing the foreign eggs and removing them from the nest. Laying eggs by cuckoo (koel) in crow's nest is brood parasitism. Cowbirds also show brood parasitism.

Coevolution is caused by the selection pressures that each of the two species exerts on the other. Since there has been no coevolved herbivore with prickly pear cactus, it became abundant in Australia.

Prey species have evolved various defenses to lessen the impact of predation. For plant, herbivores are the predators. Plants have evolved an astonishing variety of morphological and chemical defences against herbivores. Calotropis produces highly poisonous cardiac glycosides and that is why we never see any cattle or goat browsing on this plant. A wide variety of chemical substance that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.) are produced by them actually as defences against grazers and browsers.

Two or more species with closely similar niche requirements cannot exist indefinitely in the same area as sooner or later they come into competition for possession of it. This is called as Gause's competitive exclusion principle, which states that an ecological niche cannot be simultaneously and completely occupied by established populations of more than one species. Two species can live in same habitat but not in the same niche. More similar the two niches are, severe the competition is

When species 1 was kept with species 2, its number of individuals fell drastically while when species 2 was kept with species 1, its number of individuals was not much affected. Hence, both species are affected by inter-specific competition but species 2 is less affected.

Growth rate = Birth rate - death rate 
For country P, it is 10/1000. For country Q, It is 15/1000
For country R, it is 17/1000. For country S, It is 7/1000
Hence, country S has the least population growth rate 

A can be either predation or parasitism since one species is harmed and the other is benefitted. B is commensalism since one species is benefitted and other has no effect on it. C can be amensalism since one species is harmed and the other has no effect on it.

Bacteria decompose the organic matter which is not really living. Hence, it cannot be considered as a prey predator relationship.